Such a case exists that an authentication client residing as a subordinate under a certain authentication relay moves under another authentication relay after executing a network access authentication process with an authentication server via the authentication relay. In the case of moving to another authentication relay, a re-authentication procedure (update of the authentication) is to occur under another authentication relay.
Even when both of a certain authentication relay and another authentication relay belong to the same authentication domain, however, the re-authentication process may get into a failure. This case entails redoing the authentication process from the beginning under another authentication relay, and such problems arise that a processing load on an authentication relay increases, a data size (traffic) across a network rises, and communication of the authentication client resume with a delay. Accordingly, also when the authentication client moves to another authentication relay, it is desirable to ensure the re-authentication process. Further, even in a case where the authentication client does not move to another authentication relay, it is desirable to enable the re-authentication process to be executed while decreasing the load on the authentication relay.